CHAPTER V 

 THE ENDOSKELETON 



". . . our ' physic ' and ' anatomy ' have embraced 

 such infinite varieties of being, have laid open such 

 new worlds in time and space, have grappled, not 

 unsuccessfully, with such complex problems, that the 

 eyes of Vesalius and of Harvey might be dazzled by 

 the sight of the tree that has grown out of their grain 

 of mustard seed." 



THOMAS HENRY HUXLEY, in his essay: On the 

 advisableness of improving natural knowledge. 



AN endoskeleton or internal framework for the support of 

 the muscles and the protection of the viscera is one of the dis- 

 tinguishing characteristics of vertebrates, for with the excep- 

 tion of a few sporadic cases in which internal skeletal parts 

 occur, invertebrates are without such a system. The verte- 

 brate endoskeleton is a part of the connective tissue system of 

 the body and, in its usual sense, includes only bone and cartil- 

 age, although both developmentally and physiologically the 

 associated ligaments and other connective tissues belong with 

 the former. 



Primarily the endoskeleton consists of three systems, 

 originally distinct from one another, the axial, the visceral, and 

 the appendicular. The axial__ includes the vertebral column 

 and a large part of the skull ; the visceral includes the lower 

 jaw, certain elements in and about the upper jaw, the hyoid 

 apparatus, and the branchial or gill arches ; and the appendic- 

 ular includes the shoulder and hip girdles and the skeleton of 

 the free limbs. 



Of these, the axial is the oldest, and is represented in its 

 simplest form by the notochord, although this organ soon 

 yields in functional importance to the connective tissue sheath 

 which enwraps it, from which, in higher forms, the main ele- 



122 



